This invention relates generally to a controller of the type suitable for activating a switch device of an electronic type such as may be used in a combustible fuel burner. More specifically, this invention relates to a microprocessor-based controller apparatus for executing a linear program in synchrony with a recurring signal coupled to the reset terminal of the microprocessor. The preferred embodiments of the invention employing the microprocessor-based controller will be described with reference to combustible fuel burner systems, however the invention is not so limited. The invention is in fact applicable in any application where a high reliability microprocessor-based control circuit is desired.
A variety of gas ignition controls are known and generally include apparatus constructed of discrete components for effecting a single, predetermined control sequence tailored to a specific application. Such gas ignition systems typically include a relay which is energized in response to a thermostatic demand for heat and for actuating a pilot gas valve, a spark generating circuit for igniting the pilot gas either coincidently with the energization of the pilot valve or within a predetermined time thereafter, a flame-sensing probe for detecting the presence of a pilot flame and an output relay section for energizing a main fuel valve upon a detection of the pilot flame. Refinements of this fundamental system may include a pair of electromagnetic relays which are required to be operated in a particular sequence upon each reception of a thermostatic demand for heat. The sequential operation of these relays is effective in verifying the integrity of certain circuit components. Examples of such gas ignition systems are shown and described in U.S. Pat. Nos. 4,077,762 and 4,178,149. While these gas ignition systems have heretofore been satisfactory, they have failed to appreciate the manner in which a microprocessor may be utilized therein to continually monitor and verify the integrity of certain circuit components and to provide a degree of application flexibility heretofore unknown.
In particular, these earlier devices have failed to appreciate how a microprocessor-based control apparatus may be programmably configured with any one of a variety of control strategies whereby the apparatus may be readily adapted to a variety of gas-fueled furnaces, each presenting a different control requirement.
An example of a microprocessor-based controller is disclosed in U.S. Pat. No. 4,581,697, assigned to the assignee of the present invention. This microprocessor-based apparatus is an attempt to control the operation of a combustion fuel burner and to make use of a synchronizing signal for monitoring the integrity of certain circuit components, which may incorporate one or more input and output stages and which is adaptable to conduct its control sequences in accordance with predetermined lapses of time rather than upon the occurrence of predetermined events. However, in a combustion fuel burner and in other applications requiring a high degree of reliability it is necessary to be able to predict and/or limit the possible failure modes of the system. While in conventional systems described above this can be accomplished, heretofore this has not been the case with microprocessor-based systems. Conventional microprocessor-based control systems, driven by software, can theoretically have an infinite number of unpredictable failure modes. Accordingly, a microprocessor-based control system offering a high degree of reliability and readily adaptable to a wide variety of control strategies would be a distinct advance in the art.